1. Field of the Invention
The disclosure relates in general to automatic gain control (AGC) systems. In particular, the disclosure relates to circuits and methods for AGC systems with feedforward and feedback loops.
2. Description of the Related Art
The use of receivers in wireless systems such as radio and cellular communication systems is well-known in the art. A conventional technique for demodulating received signals is the use of a linear demodulator in conjunction with an automatic gain control (AGC) circuit to keep the input signal within the input range of the demodulator and/or within the input range of ADCs (analog to digital converters). An AGC circuit maintains an output within a linear operating region by adjusting the gain of an amplifier via feedback.
FIG. 1 shows a conventional AGC system. The AGC system comprises a voltage or current variable gain amplifier 10, a power computation processor 12 and a comparison circuit 14. An input signal 16 input to the AGC circuit is coupled to the variable gain amplifier 10. The power of output signal 18 is measured by the power computation processor 12 which performs an average or peak power measurement. The measured power is compared with a predefined value in the comparison circuit 14 which generates a gain control signal 19 corresponding to the difference in power level. The error signal 19 acts as a negative feedback and controls the gain of the variable gain amplifier 10. In response to the error signal 19, the variable gain amplifier 10 controls the magnitude of the output signal 18 with reference to the input signal 16. The AGC circuit maintains the output signal 18 within the linear operating region of the receiver and ADCs (not shown) employed to convert the analog signal to digital form. While AGCs obviate input overloads, the individual components within the AGC circuit contribute their own distortions.
In addition, whether implemented using analog components or some combination of digital and analog components, AGC arrangements provide for the use of other receiver components, such as ADCs, having more limited dynamic range than an overall desired operating dynamic range of a receiver. For example, if a receiver with digital signal processing functions operates in an environment where input signal level has a large dynamic range, either AGC or high resolution ADCs must be used. Unfortunately, the use of high resolution ADCs increases the manufacturing cost and power consumption of the receiver. Although AGC enables use of lower resolution ADCs, the aim of AGC is to maintain a scaled signal within a desired dynamic range, such that the amplitude of the scaled signal output from a variable-gain amplifier or gain stage does not vary significantly.